Shortcut-connected Expert Parallelism for Accelerating Mixture-of-Experts

Expert parallelism has been introduced as a strategy to distribute the computational workload of sparsely-gated mixture-of-experts (MoE) models across multiple computing devices, facilitating the execution of these increasingly large-scale models. However, the All-to-All communication intrinsic to expert parallelism constitutes a significant overhead, diminishing the MoE models' efficiency. Current optimization approaches offer some relief, yet they are constrained by the sequential interdependence of communication and computation operations. To address this limitation, we present a novel shortcut-connected MoE (ScMoE) architecture with an overlapping parallel strategy, which effectively decouples communication from its conventional sequence, allowing for a substantial overlap of 70% to 100% with computation. When compared with the prevalent top-2 MoE architecture, ScMoE demonstrates training speed improvements of 30% and 11%, and inference improvements of 40% and 15%, in our distributed environments with PCIe and NVLink hardware, respectively, where communication constitutes 60% and 15% of the total MoE time consumption. Building on the ScMoE architecture, we further implement an expert offloading strategy to facilitate memory-limited inference, optimizing latency through the overlap of expert migration. Additionally, extensive experiments and theoretical analyses indicate that ScMoE not only achieves comparable but in some instances surpasses the model quality of existing approaches.